1. Field of the Invention
This invention pertains broadly to the area of spectroscopic systems and imaging spectrometry where multiple images of an object are formed corresponding to different spectral components of the object. More specifically, the invention relates to a novel imaging spectrometer designed to acquire simultaneous, spectrally-discrete, two-dimensional images in real time while maintaining the spatial integrity of the image without scanning mechanisms or mathematically intensive reconstruction/registration algorithms.
2. Description of the Related Art
Spectroscopic systems are optical systems that allow for the determination of the spectral (wavelength) composition of objects and scenes. Generally, these systems collect the total energy coming from the object. The wavelengths that comprise the collected energy are separated with the use of a dispersive element employing refractive means such as a prism or diffractive means such as a grating. After passing through one of these dispersive elements, the different wavelength components of the wavefront propagate in different directions and their intensities are recorded by a one-dimensional array of detector pixels.
Fairly complicated spectroscopic systems have been developed in the prior art. For example, U.S. Pat. Nos. 5,149,959 and 5,276,321 describe multichannel systems which can detect the wavelength composition of objects from the visible to the infrared. However, these systems are limited by their inability to record the spectral data without losing spatial information.
Other spectroscopic systems exist that employ interferometric methods for determining the spectral content of an object or scene. The most notable systems are the Fourier transform spectrometer (U.S. Pat. Nos. 5,251,008 and 5,561,521) and the Fabry-Perot spectrometer (U.S. Pat. Nos. 5,461,477, 5,225,893, and 5,059,026). The Fourier transform spectrometer is a Twyman-Green interferometer, which can ascertain the spectral content of a point source. A positive lens collimates the light from the point source before it enters the interferometer. Either the test arm or reference arm mirror is scanned along the optical axis with the intensity being detected at each scan position. Taking the inverse Fourier transform of the envelope of the detected signal yields the spectral intensity of the object as a function of frequency or wavelength.
The Fabry-Perot system is also an interferometric system employing two highly reflective mirrors to form an optical cavity that functions as a spectral filter. Collimated light entering the system undergoes multiple reflections within the optical cavity. Only the particular wavelength for which all the multiple reflections interfere constructively pass through the optical cavity and is recorded by a detector. The particular wavelength that is passed by the optical filter depends on the distance between the two highly reflective mirrors. As this distance is changed, the wavelength passed by the filter also changes. Thus, the bandpass of the Fabry-Perot system is a function of the lateral separation of the mirrors. Therefore, as one mirror is scanned along the optical axis, effectively changing the distance between the mirrors, the bandpass is changed and the different spectral components of the source are recorded sequentially by the detector.
While standard spectrometers are excellent devices for determining the spectral composition of light emanating from an object, they are unable to maintain the spatial integrity of the object in any dimension. Typically, they just collect the total energy of the object and are not capable of determining its spectral content on a pixel by pixel basis. Thus, several systems have been developed to deal with this problem.
The first subset of imaging spectrometers is the one-dimensional scanning system. The standard representative of this category includes an imaging component that forms the image of an object onto a slit aperture. The slit aperture serves to allow a one dimensional cross-section of the two dimensional image to pass through the rest of the optical system. The light emanating from this one-dimensional line image is usually collimated by a lens before it strikes a dispersive element such as a grating or prism. The dispersive element is arranged such that the dispersion of the light is in a direction perpendicular to the line image. The dispersed light is then imaged onto a two-dimensional detector array with another lens such that the detector records one spatial dimension and a spectral dimension.
U.S. Pat. Nos. 4,743,112 and 5,260,767 disclose elaborate examples of this type of system. While the approach is a definite improvement over the basic spectrometer, it still requires scanning of the slit aperture in order to obtain spectral information of a two-dimensional image. Any scanning operation precludes the simultaneous recording of the information which is necessary in many applications.
One-D scanning imaging spectrometers need to scan a spatial dimension in order to record spectral information of a two-dimensional image. Similarly, 2-D scanning systems record information for two spatial dimensions and have to scan in order to acquire spectral information. The Fourier transform (FTS) and Fabry-Perot interferometers discussed above are capable of performing imaging spectrometry and determining the spectral composition of an object on a pixel by pixel basis. However, there are certain limitations imposed by the geometry of these systems. In both cases, the system field of view is severely restricted. For the Fourier transform interferometer, the length of the system, combined with the small size of the mirrors, restricts the f-number and the field of view because optical rays will not propagate through the system for large angles. Therefore, the number of pixels that can be used is limited. Another problem arises with respect to image registration. Two-dimensional images are acquired as one of the mirrors is scanned. Problems associated with scanning, such as mirror jitter, uneven scanning, or mirror walking, create registration problems between the images in the different spectral bands. Finally, the FTS is computationally intensive requiring N Fourier transforms to reconstruct the spectral signature on a pixel by pixel basis for a detector with N pixels. These operations are very time consuming.
The Fabry-Perot interferometer is also limited to a small field of view because of two main effects. First, the light coming from the source undergoes multiple reflections within the mirrored optical cavity before emerging from the system. When the incident light comes from an off-axis point on the object, it enters the cavity at an incident angle other than zero. Consequently, as the light undergoes multiple reflections, it will walk along the mirrors and eventually leak out of the cavity. The result is that the energy throughput of the system decreases as the field increases. The second problem with the Fabry-Perot system is that bandpass varies with field. Since the effective mirror separation changes with field angle, so does the filter bandpass. To minimize the spectral variation from the center to the edge of the field, the field of view has to be small. However, this limits the number of pixels that can be obtained. Moreover, since scanning is necessary, two-dimensional spectral images also cannot be obtained simultaneously.
In addition to these problems of image registration and simultaneity, systems that employ scanning to acquire the spectral composition of an object have difficulty maintaining a high signal-to-noise ratio. This is not only a problem for the Fourier transform and Fabry-Perot interferometers, but also for electrically scanned systems such as liquid crystal systems and acousto-optic tunable filter-based imaging spectrometers (for example, U.S. Pat. No. 5,216,484), which have the additional problem of relatively low transmission. The need for scanning can be avoided by using tomographic-based methods for imaging spectrometry tasks. However, this technique is computationally intensive, requiring the determination, mathematically, of a system matrix that is usually application specific. Thus, since the system matrices need to be reasonably small for computational reasons, tomographic techniques are not capable of providing high spatial resolution.
The 2-D scanning imaging spectrometers discussed above tend to be used for hyperspectral imaging where a large number of spectral bands are necessary (i.e., high spectral resolution). In many applications, however, only a few spectral bands are needed. U.S. Pat. No. 4,134,683, for example, discusses the use of multiple channels where each channel consists of a lens system, a spectral filter and a detector array. Each channel is aimed at the same object. This approach has the crucial disadvantage that the components within each channel have to be properly aligned such that corresponding pixels on each detector are mapped to the same point on the object. With multiple optical systems, it is very difficult to achieve this alignment because of separate tilt, decenter, and boresight errors. In addition, each lens system will have different imaging properties because of differing aberrations from lens to lens causing each of the images to differ.
A second approach (used in U.S. Pat. Nos. 4,268,119, 4,084,180, 4,072,405 and 4,916,529) uses a single optical system in conjunction with a multiple prism assembly. The prism assembly is used to split the incident light into separate beams propagating in different directions. Each beam path has a different spectral filter before the detector array to achieve multispectral imaging. Cube beamsplitters can be used instead of prism assemblies (U.S. Pat. No. 5,414,458). The prism/beamsplitter approach also has some serious drawbacks, though. If the incident beam is not perfectly collimated when it enters the prism assembly, the prism system will introduce a number of aberrations reducing resolution and degrading imagery. Even if the beam is collimated, spectral dispersion caused by the prism will reduce resolution and lead to image registration problems. Finally, due to total internal reflection, using prism assemblies in transmission does not allow the use of the optical system at low f-numbers or large fields of view.
A third approach utilizes a series of dichroic beamsplitters to send the incident light propagating in different directions (U.S. Pat. Nos. 4,281,339 and 4,531,054). The dichroic beamsplitter has an advantage over the prism assembly in that it not only splits the incident light but it does so in a spectrally selective manner without introducing aberrations or significant spectral dispersion. That is, it splits the incident beam by passing one set of wavelengths and reflecting another. Using a set of dichroic beamsplitters and multiple detectors, a multispectral imaging system can be assembled.
These approaches allow for the simultaneous acquisition of spectrally discrete images; however, image registration still remains a difficult problem. Every detector and its electrical readout circuitry have specific noise properties associated with the system. Thus, the use of different detectors means that each image will have different noise and gain properties making registration more difficult.
U.S. Pat. No. 4,650,321 discusses a multiple detector system where two detector arrays are used in combination with a dispersive imaging system. In this approach, an a focal telescope system is utilized with a concave reflective grating to form an image of the object on one detector. The image on the first detector corresponds to the undiffracted (zero order) beams. On the other detector, the first order diffracted beams are focused forming spectral images of each point on the object. If the points on the object are spaced close together, then the spectral images of each point will overlap and the spectral information will be lost. As a result, this system can only work properly if the 2-D scene being viewed consists of a small number of well-separated point sources, like stars, for example. This system will not function properly when viewing a standard 2-D scene.
To overcome the problems associated with multiple detector systems, attempts have been made to achieve the formation of multiple, spatially identical, but spectrally discrete images on a single film plane or detector array. U.S. Pat. No. 3,720,146 describes the use of four lenses arranged in a parallelogram configuration to simultaneously produce four images on a film plane. U.S. Pat. No. 5,479,015 also implements multiple focusing members to form a plurality of identical images on a single detector array. The use of a film plane has obvious disadvantages when compared to a detector array and the use of multiple lens systems introduces the same problems already discussed with respect to U.S. Pat. No. 4,134,683.
U.S. Pat. No. 4,141,625 discusses the use of two partially reflecting mirrors in combination with a single lens system to create two images of an object on a single detector array. Tilting these mirrors in both the x and y directions allows for the separation of the images at the detector plane. While this design achieves the objective of creating multiple identical images, the use of reflective mirrors leads to image inversion. One-dimensional imagery is fairly straightforward in this application. Two-dimensional imagery has some problems because the reflective mirrors are not specifically located in a pupil plane. As a result, each channel does not contain the same amount of energy and this difference is exaggerated as the field increases. In addition, vignetting (light loss, as well understood in the art) can be significant. This configuration also does not address any way to form images which have different spectral components because the mirrors are not spectrally selective. Finally, the reflective mirrors are situated such that the light incident upon them is diverging. This has the disadvantage of being radiometrically inefficient and a high level of background noise will be present on the detector because the mirrors are reflecting not only light from the object, but also background light through the lens. The combination of radiometric inefficiency and high noise leads to a reduction of the signal-to-noise ratio.
U.S. Pat. No. 4,272,684 attempts to address the problem of radiometric efficiency and even uses a reflective prism configuration to function as a beamsplitter. However, this configuration suffers from the same problems as the previous system, namely an inability to acquire more than two images, nonuniformity of the images, and an inability to acquire these two-dimensional images without incurring loss in throughput with field because the reflective prism is not located in a pupil plane. As with the previous system, this approach has no means for producing spectrally discrete images.
Filter wheel systems have also been used as a means of obtaining spectral images using a single detector (U.S. Pat. No. 5,587,784). In these systems, a standard imaging scheme is used to image a 2-D object onto a detector array. A filter wheel assembly is placed in the optical path such that one of the filters transmits a fixed set of wavelengths. If a different set of wavelengths is to be passed, the wheel is rotated and a different filter is placed in the optical path. By rotating the wheel, different spectral images are obtained. Thus, it is clear that simultaneous spectral images cannot be acquired with this approach.
U.S. Pat. No. 4,933,751 describes a tri-color separating system which uses four dichroic beamsplitters to form three separate color images right next to each other at an image plane. An immediate problem with this configuration is that the filters are not located in a collimated space. Since the filters are located in a space where the incident beams are converging cones of light, the spectral filtering of the light will not be constant over the cone. This effect is common in this configuration because the bandpass of a spectral filter is sensitive to the angle of incidence. As a result, true spectral discrimination for each point in the object is not possible.
U.S. Pat. No. 4,786,813 discloses a method for producing two-dimensional, spectrally discrete images on a single detector array which employs a segmented concave mirror. This segmented mirror has the dual function of separating the beams originating at the object and focusing the beams onto the detector to form the images. While this system achieves the desired objective, the properties of the design lead to poor optical performance in all but a handful of situations. Since the imaging system is only comprised of a single spherical mirror, aberrations reduce resolution and degrade imagery. This is not desirable, especially in microscopy applications. If any reasonable field of view is being imaged, spherical aberration is a problem along with coma, astigmatism and field curvature. Since the spherical mirror is tilted to form the images on the detector, all of the field aberrations also exist on-axis. Again, the location of the spectral filters presents a problem because they are located in a space where the incident beams are converging cones of light.
Finally, some prior-art systems attempt to perform multi-spectral, two-dimensional imaging on a single detector array without scanning, but each system again has serious limitations. U.S. Pat. No. 5,024,530 discusses a telecentric, filtered imager capable of producing only two spectral images of an object. This configuration has a number of disadvantages. First, the incident beams of light need to be filtered at two different planes of the lens system. Second, while the first filter plane is located in a telecentric space, it is not located in a collimated space. As a result, all the associated problems of having a non-collimated beam passing through an interference filter are prevalent. Third, while the second filter plane is in a collimated space, the filters are not removable, making filter substitution more difficult. Finally, the beam separation assembly consists of a triangular prism (i.e. two facets) used in transmission. That is, refraction of the incident light is the mechanism used for separating the beams, resulting in inherent problems since refraction is a wavelength dependent phenomena. Therefore, the beams will not only be separated spatially, but in each of the spatially separated beams there will be a spectral separation due to the optical dispersion of the prism. This optical dispersion will lead to a smearing effect at the detector plane reducing resolution, degrading imagery, and creating image registration problems.
U.S. Pat. No. 5,642,191 discusses a very similar approach and suffers from many of the same drawbacks. U.S. Pat. No. 5,526,119 avoids the limitation of two-band imaging with the use of multi-faceted prisms to obtain more images. However, since the prism is again used in transmission, all the problems related to optical dispersion associated with refraction remain. This system is even less flexible with respect to filter replacement because the spectral filters are described to be attached mechanically or through adhesion to or deposition on the prism itself. In addition, the manner in which the field stop is used leads to images on the detector which have dead space between them. The lack of contiguous multiple images does not allow for the use of the full resolution or field of view of the detector array which will be problematic in many applications.
Therefore, there is still a need for a multi-spectral two-dimensional imaging spectrometer that is capable of real-time imaging without scanning and/or computation. This invention is directed at providing an apparatus and a related spectrometric approach to fulfill that need.